Swimming is not only one kind of wonderful sports but also one kind of activities for taking a summer holiday. More and more aquatic games are made and designed for fun, such as holding one's breath under water, walk race in water, treasure hunt under water, ball games in water and so on.
A conventional game of treasure hunt under water is that a coin is thrown in water and swimmers dive into water to look for the coin. The guy who first finds the coin is the winner. However, it is monotonous to look for a coin under the water and it is inconvenient to pick up a coin under the water since the coin is quite small.